l-n-Menthol has been widely used as a cooling agent on the world-wide scale of thousands of tons per year in drugs, toothpaste, tobacco, chewing gum, confectionery, beverages, cosmetics, etc.
Natural l-n-menthol can be obtained by cooling peppermint oil obtained by steam distillation of Japanese peppermint (Mentha arvensis) and recrystallizing the precipitated l-n-menthol from peppermint white oil as a solvent. Processes currently adopted for synthesizing l-n-menthol on an industrial scale include a process starting with d-citronellal (Takasago process; see Indo Motoichi, Koryo, No. 177, pp 33-47 (1993)) and a process starting with thymol (Haarman Reimer GMBH DE).
Since l-n-menthol has a melting point of 42.degree. to 44.degree. C. and is solid at room temperature, synthetic l-n-menthol products for the market are prepared by distilling crude l-n-menthol, flaking the resulting liquid l-n-menthol having a temperature above the melting point in a flaking machine, and packaging the resulting flaky menthol usually in 50 l-volume fibredrums. However, the loss of menthol due to sublimation during packaging reaches 2%, and the poor workability in this packaging operation increases the cost of commercialization. Additionally, the form of the package and the small bulk density of the flakes also increase the cost of packaging and transportation. All these factors have increased the cost of menthol.
On the other hand, handling of flaky menthol involves a problem of working environment. That is, workers on flakes of menthol are to be exposed to highly concentrated menthol vapor, which would be too strong irritation in long-time working.
Further, it has been pointed out that menthol flakes are apt to cake partly when preserved at high humidity and high temperature (below the melting point, e.g., 25.degree. to 35.degree. C.) for a long time, and cases are sometimes met with in which a user finds the product no more usable practically. Therefore, the development of the process for improving these problems has been desired.